Recently, developing machines called mini labs or digital labs have been popularized. They do not directly print an image from film onto paper, but rather, perform various steps in processing a film image by using a lens and a solid-state image pick-up element, such as a CCD array. An image is indirectly copied onto paper by projecting the film image using the lens onto the CCD array, where it is captured and used to modulate a laser beam that writes the image onto paper as is done in a laser printer. In such developing machines, an image readout optical system having higher resolution for imaging the film image onto the solid-state image pick-up element has been desired in order to keep pace with the rapid development of higher resolution CCD arrays.
When a color original is read, for example, by a three-line CCD array, it is desired that chromatic aberrations be favorably corrected so that the imaging positions for the red (R), green (G) and blue (B) color components are incident onto the appropriate line of the three-line CCD array. More specifically, the colors must be imaged so that they have high contrast by reducing the axial chromatic aberration and the lateral color of the R, G, and B color components in order to achieve proper registration of these color components over the entire image field onto the respective line of the detectors. Even if the amount of axial chromatic aberration is small, the positions of the image points will deviate with a change in color. If the R, G, and B image positions are mis-registered with respect to the position of the sensors of the three-line CCD array, full color images having reduced contrast will result even if high contrast performance is obtained separately for each color. Thus, mechanisms which detect the focus separately for each color are needed in order to maintain proper registration of the focus positions.
In addition, there is a requirement that very fine blemishes or dust particles on the surface of the film that form image artifacts by being incorporated as image information onto the paper, be minimized. Corresponding to this requirement, an attempt has been made to detect very fine blemishes or dust particles using near-infrared light and to remove these unnecessary images by electronic image processing.
Accordingly, it is desired that a color image readout lens provide a high resolution image not only in the visible region for obtaining the image information, but also in the near-infrared wavelength region.
If f is the focal length of a lens and xcex2 (xcex2 less than 0) is the magnification of the lens, the amount of axial chromatic aberration xcex94S is proportional to (1xe2x88x92xcex2)2xc2x7f. Thus, the longer the focal length f or the greater the absolute value of (1xe2x88x92xcex2), the larger is the amount xcex94S. This property is significant, and the correction of chromatic aberration on the optical axis is impossible unless a glass having anomalous dispersion is used. It is particularly difficult to make the R, G, and B color components have high-contrast over an expanded region. Therefore, a glass of high anomalous dispersion must be effectively used in order to correct large axial chromatic aberrations.
The higher the anomalous dispersion of the glass, the higher the correction effect. And, the stronger the refractive power of the lens, the stronger the correction needed. On the other hand, if the refractive power is too strong, the occurrence of other aberrations increases and this causes a deterioration in the quality of the image.
It is desirable to correct the axial chromatic aberration by ensuring appropriate values for the sum of the products xcexa3xcfx86ixc2x7xcex4xcex8i, where xcfx86i is the refractive power (i.e., equals 1/fi) and xcex4xcex8i is the anomalous dispersion of the ith lens element.
Many proposals for making a lens achromatic have been set forth, such as the one described in Japanese Laid Open Patent Application H4-311912. However, almost all of these proposals form reduced images having a magnification in the range xe2x88x920.1 to xe2x88x920.2 of that used, for instance, in a scanner. In a developing machine as described above, it is difficult to correct large axial chromatic aberration when approaching a magnification near unity. For example, it is difficult to obtain good performance when the magnification is about 0.8.
On the other hand, a technique described in Japanese Laid Open Patent Application H10-325921 has been used for correcting aberrations in the near-infrared region as well as in the visible region. However, this technique does not use a diaphragm at the center of the lens system, as is commonly done in order to obtain symmetry and thereby reduce certain aberrations so as to provide good optical performance. Therefore aberration fluctuations over the range of magnification increase, making good optical performance difficult to obtain over a wide range of magnification.
The present invention relates to a lens for reading a color image or a color original, and to a device which uses such a lens. More particularly, the object of the present invention is to provide a color image readout lens which reduces a negative film image or positive film image by a magnification factor mf, with mf being in the range xe2x88x920.3xe2x89xa6mfxe2x89xa6xe2x88x920.85, which has an f number FNO in the range of 7.3xe2x89xa6FNOxe2x89xa610.2, and which simultaneously provides good visible and near-infrared imaging resolution. In addition, the invention is to a device which uses such an image readout lens.